President Obama spent yesterday afternoon meeting with Senate Democrats. The event was held in a meeting space at Nationals Park. That's the baseball stadium in Washington. It was billed as an issues conference. And certainly one of the big issues for the president's party is the possibility that it could lose its majority in the Senate this fall. Democrats are defending 21 seats in 35 Senate races this year.

A number of those incumbents are vulnerable in both red states and purple states. That might be why Obama brought along his party's designated hitter, former President Bill Clinton.

This is a year when the Democrats are at risk, particularly in the Senate. Right?

MARA LIASSON, BYLINE: That's right. The Republicans only need net six pickups to get the majority in the Senate. And history tells us that second-term midterms are very hard on the party that has the White House. They generally lose an average of about five Senate seats. And the map this year is very bad for Democrats. There are a lot of red states that are up for grabs. There are three open seats in red states - West Virginia, Montana and South Dakota - where Democrats are retiring. And those are considered excellent pickup opportunities for Republicans.

Then there are four endangered Democratic incumbents in red states: in Arkansas, Alaska, North Carolina and Louisiana. And Republicans are optimistic they can expand the map even further to Michigan, Iowa, Colorado and New Hampshire. So they think they have a pretty big pool to fish in for six seats. Now, they'll definitely get some of them. The question is: Can they get the six they need to get the majority from the Democrats?

INSKEEP: OK. So the Republicans need a big win in order to capture the Senate. You're saying that it seems to be in range, they at least have places to fish for that. At the same time, Democrats might be looking to win a couple of those seats back in other places. Might they?

LIASSON: Well, they might. But only one Senate Republican who's up in 2014 comes from a state that President Obama carried, and that's Maine, and that's Susan Collins. And she is not endangered. But Democrats' hopes do spring eternal. There is an open seat in Georgia, where Michelle Nunn - daughter of Sam Nunn - is running for that seat.

In Kentucky, Senate Minority Leader Mitch McConnell is getting a vigorous primary challenge from a Tea Party conservative. And then he faces Alison Lundergan Grimes, who's running an aggressive campaign. But any time you take on a party leader - especially one as wily as Mitch McConnell - it's a very long shot. But Democrats are keeping hope alive.

INSKEEP: So, Republicans have a lot of advantages, here, but let me ask about another thing, Mara Liasson: This is maybe a good time to remember that in both 2010 and 2012, Republicans seemed positioned to gain more seats than they actually did. But in primary fights, they nominated candidates who were considered too conservative or too extreme, depending on who you asked.

LIASSON: Right. And they did snatch defeat from the jaws from victory. And the question is: Will they do that again? We won't know the answer till after the Republican Senate primaries are done. But the Republican Party this year is going to great lengths to avoid those mistakes again. There are a lot of establishment Republicans, the Chamber of Commerce pouring money into these primaries to make sure that the most electable general election candidate wins the primary.

INSKEEP: So this is what Democrats spent part of yesterday talking about in the bowels of National Stadium in Washington, D.C. Of course, the key Democrat here is the president of the United States, who's not on the ballot, but is there anything that he can do to help his side?

LIASSON: Well, there are some things he can do, not a lot of things. But he can raise money, and he's been doing that. He can try to get his approval ratings up, because historically, midterm election performance does track a president's approval ratings. But the problem for President Obama is the states where Democrats are in trouble - like Louisiana and Arkansas and Alaska - are not states where he can help Democratic candidates by campaigning in person.

The midterm electorates there are more white and rural than in presidential years. And in most of those states, he didn't even win the Democratic primary for president. North Carolina could be an exception. They have a larger African-American population that traditionally doesn't turn out in off-year elections. But in the Virginia governor's race we just had, the Democrats figured out how to almost replicate a general election - presidential election turnout. And they're hoping they can do that in North Carolina.

INSKEEP: Of course, there is a designated hitter president that President Obama could send into some of those states you mentioned, Mara.

LIASSON: Yes, there is. His name is Bill Clinton, the former president. And yesterday, he was with President Obama at the Senate Democrats' retreat. He can go into the states that President Obama can't. He's already campaigned for Mark Pryor in Arkansas, where, of course, Bill Clinton is from and was the governor for a long time. And I wouldn't be surprised to see Bill Clinton campaigning for endangered red state Democrats elsewhere.